Chlamydia trachomatis is a bacterial pathogen that generally infects the ocular and genital mucosal epithelium and produces uncomplicated infections such as conjunctivitis, urethritis, and cervicitis. However, these can progress to chronic infections that provoke severe inflammatory responses that lead to blindness and infertility. The objectives of this project are to characterize the antigenic properties of the chlamydial 60kDa heat shock protein (hsp60), which has previously been shown to stimulate immune response responses that contribute to the pathology of chlamydial disease, and to use in vitro models of chlamydial infection to study chlamydial persistence. The antigenic structure of the hsp60 was characterized by defining immunogenic regions of the polypeptide using monoclonal antibodies and sera from patients having chlamydial infection. Ten anti-chlamydial hsp60 mAbs were prepared and their reactive epitopes precisely mapped. Most mAbs reacted with linear epitopes located at the amino or carboxyl termini of the hsp60 polypeptide. The mAbs were useful for detection of chlamydial inclusions by indirect immunofluorescence, and for the purification of recombinant hsp60 by immunoaffinity chromatography. Patients with post-chlamydial reactive arthritis and women having had ectopic pregnancies had high titered anti-hsp60 serum antibody. The anti- hsp60 response was predominantly chlamydial specific (i.e. did not crossreact with the E. coli hsp60 homologue) and was localized to the amino and carboxyl termini, and to a polypeptide containing amino acids 200 to 300. Further analyses are being performed to determine if the anti-hsp60 response differs among patients with acute or chronic chlamydial disease. The antigenic stimulus for the chronic inflammation associated with blindness and infertility is unknown, but could result from persistent infection. To determine factors that can lead to persistent chlamydial infection we used an in vitro model of chlamydial infection. Interferon- gamma (IFN-gamma), a cytokine released from lymphocytes in response to antigenic stimulation, was found to cause persistent chlamydial infection in vitro. IFN-gamma inhibited the production of infectious progeny, despite the presence of intracellular chlamydiae. Analysis of infected cells treated with IFN-gamma revealed morphological changes in chlamydiae characterized large abnormal intracellular forms. These atypical forms were found to exhibit differential expression of chlamydial antigens. The chlamydial hsp60 was found to be in abundance, while only a paucity of the major outer membrane protein (MOMP) was detected. Despite altered morphology and antigen expression, infectious progeny were recoverable when IFN-gamma was removed. If IFN-gamma has a similar effect in vivo, then persistently infected cells containing an abundance of hsp60, a deleterious immune target, and a paucity of MOMP, a protective immune target, could serve as depots of antigen for the stimulation of chronic inflammation that precedes blindness or infertility.